Shelf unit



FIG. 1 is a front perspective view of a shelf unit showing my new design;

FIG. 2 is a left side view thereof, a view from the right side being a mirror image;

FIG. 3 is front plan view thereof, a view from the rear being a mirror image thereof; and,

FIG. 4 is a top plan view thereof. 

The ornamental design for a shelf unit, as shown and described. 